This invention relates to an anesthesia machine, and more particularly, to a machine adapted to receive separate supplies of oxygen and nitrous oxide and to cause mixing thereof to dispense a mixture of oxygen and nitrous oxide for anesthetizing a patient. The machine is provided with the capability of varying the relative proportion of the individual gases in the final mixture yet an interlocking system prevents certain mixtures being selected, even inadvertently, by the user.
One very common means of inducing anesthesia in patients is the combination of oxygen and nitrous oxide. For certain dental procedures, that mixture alone may give sufficient anesthesia, while in more serious surgical operations, the mixture may be used for induction followed by administration of more potent inhalant anesthetics such as enflurane, marketed under the trademark Ethrane.
There are, therefore, various different techniques and corresponding apparatus for mixing the two components, oxygen and nitrous oxide, to evolve a mixture that may vary in proportions over a range.
One typical arrangement is to provide two needle valves, one of which receives oxygen and the other of which receives the nitrous oxide. Both gases are available in hospitals through built-in piping systems, or in the alternative and in the event of any failure of such piping systems, supplies are readily available commercially in cylinders. While the use of individual needle valves to provide the respirable mixture is relatively simple in operation and components, it does suffer from a potential safety problem in that a user could inadvertently create a mixture having too low a percentage of oxygen in such mixture. As is well known, the oxygen percentage in the inspired gases should not fall below about 21% by volume. Also, there must be a minimum flow of oxygen to the patient in order to meet the metabolic requirement of the patient.
Other devices have been marketed, such as shown in U.S. Pat. Nos. 3,717,177 to Glesmann and 3,739,799 to Bickford et al, however, such devices, while acceptable to maintain oxygen ratios, are generally quite complex to construct and thus are relatively expensive. In addition, they are not readily accepted by many users that are accustomed to the more conventional technique of adjusting individual needle valves to obtain the desired mixtures.